1. Field of the Invention:
This invention relates to an improved diffractive subtractive filtering technique for reconstructing black-and-white images in an optical projector using the zero diffraction order of light transmitted through a diffractive structure in which a grating-like carrier is modulated with the image information.
2. Description of the Prior Art:
U.S. Pat. No. 3,732,363 of W. E. Glenn, Jr., issued May 8, 1973, discloses a diffractive substractive filter in the form of a single, amplitude-modulated sinusoidal diffraction grating having a predetermined line spacing, which may be embossed as a relief pattern in the surface of a medium such as transparent thermoplastic film. The amplitude of the sinusoidal grating varies from point to point in accordance with recorded pictorial or alphanumeric information. More specifically, the sinusoidal grating has a predetermined amplitude depth corresponding to all "black" points of the recorded pictorial or alphanumeric information, has a zero amplitude corresponding to all "white" points of the recorded pictorial or alphanumeric information, and has respective amplitudes greater than zero but less than the predetermined amplitude corresponding to varying degress of "gray" points of the recorded pictorial or alphanumeric information.
Readout of the recorded pictorial or alphanumeric information, in U.S. Pat. No. 3,732,363, may be achieved in an optical projector by illuminating the diffractive subtractive filter with white light and imaging onto a screen only the zero diffraction order of the light which emerges from the filter. More specifically, most of the light incident on regions of the filter having the aforesaid predetermined amplitude, corresponding to black points, is diffracted into higher diffraction orders, so that only a small portion of the incident light corresponding to black points is present in the zero diffraction order which is imaged. Therefore, these black points exhibit low luminosity and appear relatively black in the image. However, light incident in the zero-amplitude portion of the filter, corresponding to white points, passes undiffracted through the filter, so that substantially all the incident light corresponding to white points remains within the zero diffraction order of light which is imaged. Therefore, these points exhibit high luminosity and appear white in the image. The relative amount of incident light diffractively subtracted from the zero diffraction order and deflected into the higher diffraction orders by respective intermediate-amplitude region of the filter, corresponding to points of varying shades of "gray", is greater than it is for the white points but less than it is for the black points. Therefore, these points exhibit intermediate luminosites and appear gray in the image.
The contrast of a black-and-white image obtainable with a diffractive subtractive filter of the type disclosed in U.S. Pat. No. 3,732,363 on reconstruction in an optical projector depends upon the ratio between the relatively low luninosity of a black image point to the relatively high luminosity of a white image point. The contrast obtainable from such a single, amplitude-modulated sinusoidal diffraction grating, of the type disclosed in U.S. Pat. No. 3,732,363, is relatively poor. However, in a paper entitled "Transmission Characteristics of Sinusoidal Phase Gratings", presented at the 1973 Annual Meeting of the Optical Society of America, Rochester, New York, C.S. Ih disclosed that zero-order contrast on reconstruction in an optical projector can be improved by the superposition of a number of sine wave gratings.
U.S. Pat. No. 3,957,354, issued May 18, 1976, and assigned to the same assignee as the present application, discloses a diffractive subtractive color filtering technique. It is disclosed in U.S. pat. No. 3,957,354, that the relative amount of incident light which is diffracted by a diffractive structure to higher orders in a given function of light wavelength (color of the light) which depends soley on (1) the particular shape of the profile (sinusoidal, square wave, etc.) of the diffractive structure and (2) the peak-to-peak optical amplitude (i.e. physical peak-to-peak amplitude multiplied by the difference between the index of refraction of the diffractive structure and that of its surroundings) of the diffractive structure profile.
One of the profile shapes considered in U.S. Pat. No. 3,957,354 is a sinusoid. It is shown that a sinusoidal profile exhibits a relatively small wavelength selectivity compared to that exhibited by a square-wave profile. Therefore, with a sinusoidal profile, it is possible to select a particular predetermined peak-to-peak optical amplitude such that substantially all the incident light at some given wavelength in the middle of the visible spectrum (i.e. at some point in the green) is diffracted into the higher diffraction orders, and still, in this case, diffract a significant, but smaller, amount of the incident light at the opposite ends of the visible spectrum (the red end and the blue end) into the higher diffraction orders. Therefore, even when the luminosity of the light remaining in the zero diffraction order is relatively at its minimum, its color is not a neutral black or neutral dark gray, but is actually a dark shade of magenta. This is true because the relative intensity of light still remaining in the imaged zero diffraction order in both the red and in the blue portions of the visible spectrum is significantly greater than that in the green portion of the visible spectrum.
Further, in the gray-scale of zero-diffraction-order-light luminosity achieved by the approach disclosed in U.S. Pat. No. 3,732,363, (i.e. varying the relative "grayness" as continuous function of amplitude of a single sinusoidal grating between zero and a predetermined maximum where the luminosity of the zero-diffraction-order light is a minimum), the color hue of the zero diffraction order is not constant, but varies because the wavelength color selectivity characteristics of a sinusoidal diffractive structure change as a function of amplitude. Thus, if black is manifested by a certain dark shade of magenta, "light gray" is manifested by a relatively light tint of some other different color. Therefore, a diffractive substractive structure of the type disclosed in the aforesaid U.S. Pat. No. 3,732,363 is not capable of producing an essentially neutral black-and-white image, when reconstructed in an optical projector.